Sleep cycles

Sleep is such a regular part of our lives that we rarely consider its nature. When we do stop to think about it, the act of sleeping can seem puzzling. While asleep, we lose consciousness and abandon control of our movements and thoughts. If such an experience were to occur during our waking lives, it would be frightening, yet we endure this process every night. We do know, however, that lack of sleep is detrimental to our health: it causes fatigue, poor concentration, and extreme emotions.

Modern sleep analysis began nearly 50 years ago. Interest grew when it was discovered that sleep consists of clearly differentiated phases. These can be determined by the observation of brain waves and general physiological activity. During an average night's sleep, four phases of sleep occur. The cycle of movement from phase one through phase four usually happens about seven times a night. Each cycle lasts approximately 90 minutes.

The four phases

In phase one of the sleep cycle, the sleeper moves from wakefulness to sleep. Phase two marks the beginning of actual sleep, where the sleeper is unaware of outside stimulation. Phase three is a gradual continuation of the transition into deeper sleep. In the final phase, phase four, we sink into an even deeper level of sleep. During this phase, the sleeper breathes more rhythmically and deeply. Heart rate and blood pressure drop, the metabolism slows, and the electrical activity of the brain is different from its waking state. When phase four is complete, the sleeper moves back through the phases, and the physiological changes are reversed: the pulse beats faster and less regularly, the metabolism and electrical activity return to their waking state, the blood pressure increases, the body often moves, and a penile or clitoral erection can occur. As these reverse changes happen, the sleeper seems to be on the verge of waking-yet it is, paradoxically, harder to wake someone at this point than during the deep level of phase four.

REM sleep

Phase four is also known as "rapid eye movement," or REM, sleep. This describes the process whereby the eyes dart rapidly from side to side under closed eyelids-and it marks the onset of dreaming. Dreams can happen in phases one, two, or three of the sleep cycle, but they are less frequent and not as vivid as those occurring in REM sleep. It is hypothesized that the rapid movements of the eye may actually represent the dreamer's observation of the events that are taking place in the dream.

In 1953, the American physiologist Nathaniel Kleitman and his student Eugene Aserinsky pioneered a clinical study of dreaming. They discovered that if a sleeper was awakened during the time when the electrical impulses of the brain exhibited certain rhythms, he or she reported dreaming at that time. The periods of brain activity and dreaming also corresponded with the occurrence of rapid eye movements. These discoveries marked the beginning of an intense period of study of dreaming and dream patterns.